This invention relates to apparatus for winding web. It relates more particularly to winding apparatus which maintains constant tension in the web during the winding operation.
A web winder is a machine which winds up the output of a web producing machine into a roll. For example, a winder may be positioned to receive printed web from a printing press. Sometimes the web winder may include a splicer so that a succession of webs of finite length can be wound up continuously on the same roll. An example of such a winder is disclosed in U.S. Pat. No. 3,756,526 owned by the assignee of the present application.
In web winders generally, it is desirable to maintain constant tension in the web span between the web producing machine and the building web roll. Consequently, the web is trained through a tension sensing device such as a force-loaded dancer positioned between the web producing machine and the web roll. Web tension upsets cause the dancer to move from a selected reference position reflecting the desired web tension. These movements are sensed and used to increase or decrease the speed of the motive means driving the winding web roll as needed to maintain the dancer at its reference position thus compensating for the tension upset. For example, if web tension increases, the dancer is moved from its reference position in one direction. This displacement is sensed and a control signal is applied to the motive means causing it to slow down. This reduces the tension in the web with the result that the dancer returns to its reference position. Conversely, if the tension in the web decreases, the dancer moves in the opposite direction causing the motive means to speed up sufficiently to return the dancer to its reference position.
Winders may be classified into two general types, namely, surface winders and center winders. In the former, a driven roll engages the outermost convolution of the building web roll. In the absence of a web tension upset, the surface speed of the driven roll remains constant as the radius of the building roll increases. In the center winder, with which we are concerned here, however, the core chucks or spindle on which the web is being wound is given a velocity and torque such as to impart a constant tension or a selected tension taper to the web. In the center winder, then, the web surface speed and therefore its tension will vary depending upon the roll radius. Consequently, a center winder drive is required to impart torque and angular velocity to the winding roll that are dependent upon the roll radius. That is, the winder drive requires increasing torque at decreasing angular velocity to maintain selected web tension and surface speed conditions.
Since material issuing from a web producing machine is ordinarily approaching the winder at a fixed velocity, the constant web tension winding requirement is equivalent to a constant winder horsepower because the horsepower is proportional to the product of web velocity and web tension. Thus, while the winding horsepower of a center winder remains constant, the drive that provides it must be capable of supplying both a much larger torque and a higher speed than is required by the surface winder in order to properly drive the roll at both roll radius extremes.
If the winding operation is performed by an electric motor coupled to the core shaft or spindle either directly or through a constant mechanical transmission ratio, then the size of the motor will be determined by the maximum torque that must be provided when the roll is full. In most instances, then, this will dictate a motor horsepower rating substantially greater than the horsepower that must actually be delivered to the core chuck or spindle and the ancillary control electronics capable of handling the larger motor.
This maximum horsepower requirement can be reduced to some degree by taking advantage of the desirability of winding the rolls with a tension taper, i.e., a higher web tension at the core than at the surface of the completed roll, and also by such techniques as field weakening in the winder drive motor. However, even the use of such measures does not suffice to overcome the need for an oversized winder drive. As an example, a web roll is typically wound with a build-up ratio of maximum diameter to core diameter of 10 or 12 to 1. An optimum tension taper may be in the range of 1.5 to 2. Motor field weakening can produce an overspeed of a large DC motor on the order of 1.5 times its full torque rated speed. However, although such a technique can provide for a constant horsepower range of up to 6 to 1, the motor size and therefore its cost is still dictated by the maximum torque requirement, which results in a motor frame size capable of 6 times the motor horsepower rating based on full field low RPM operation.
To overcome the aforesaid limitation, various types of variable ratio transmission devices have been incorporated into web winders of this general type. For example, gear shifting devices which provide a plurality of fixed ratios have been used. However, such devices involve an inherent discontinuity in the winding process when changing from one ratio to another which produces web tension upsets. Therefore, these types of devices are usually unacceptable for most continuous web winding applications.
There do exist continuous variable ratio transmissions such as the so-called Variator which avoid such discontinuities. While these have been tried in web winders, their operation has not been entirely satisfactory because they are quite difficult to control by the usual web tension sensing devices. That is, when typical tension sensors such as dancer rollers, tension transducers and the like are used, they are difficult or nearly impossible to interface with such continuously variable speed transmissions due to the slow response of the transmission's speed control. Consequently, such mechanical transmissions have not found wide application in web winding apparatus.